1. Field of Invention
Embodiments of the present invention relate generally to mask read-only-memory (ROM) devices, semiconductor devices including mask ROM devices and methods of fabricating the mask ROM devices and semiconductor device including the same. More particularly, embodiments of the present invention relate to a mask ROM device including a cell junction, which has ion implantation regions of opposite polarities and thus enables an on-cell and an off-cell to operate stably, a semiconductor device including such a mask ROM device, and methods of fabricating the mask ROM device and the semiconductor device.
2. Description of the Related Art
Mask read-only-memory (ROM) devices are read-only semiconductor memory devices to which data cannot be written. A mask ROM device includes an on-cell and an off-cell. When activated, the on-cell remains on so that electric current can be transmitted from a source region to a drain region. On the other hand, the off-cell does not transmit electric current and remains off. Each cell has a transistor structure.
The term “on-cell” refers to a cell having a transistor structure in which a channel is formed in response to an activation voltage applied to a gate electrode and thus electric current is transmitted through the channel. The term “off-cell” refers to a cell having a transistor structure in which no channel is formed and thus electric current is not transmitted. When fabricated, the mask ROM device stores predetermined data. For example, the on-cell denotes data 1, and the off-cell denotes data 0.
In the mask ROM device, it is particularly important for the off-cell to stably output data 0. Accordingly, it is important that no channel is formed—even when the activation voltage is applied to a gate electrode therein. For this reason, numerous methods of preventing the formation of a channel have been suggested. In one method, ions of a polarity opposite to that of a channel are implanted into a channel region in order to prevent the formation of the channel. However, as a design rule of gate electrodes is reduced, it can be very difficult to implant ions into the channel region.
As the channel regions into which ions are implanted are reduced, it becomes more difficult to form ion implantation mask patterns at precise locations. In addition, because ions penetrate through the gate electrode, high ion implantation energy is required. Therefore, a very thick ion implantation mask must be formed. Because a high concentration of ions of a polarity opposite to that of the channel are also implanted into a junction region, leakage current increases while a breakdown voltage is reduced.